First Model to Show Rapid Transition Toward Complete Germ-soma Differentiation

Topics relating to evolution are sure to incite debate. Anyone who talks about life progression in the context of evolution can easily raise eyebrows of the cynics.

Who would easily believe that multicellular living things like us could come from unicellular living things?

Nevertheless, those who support evolution cannot be subdued as well. If you give them a hint of doubt they would rush before you countless research findings, logical answers, and hard proofs linking to evolution. And early this year, Sergey Gavrilets, a researcher from National Institute for Mathematical and Biological Synthesis in University of Tennessee, U.S.A., would be excited to show you his math-biological model that shows how the transition of a colony of individual cells into an organized system of cells is possible. In fact, it is so fast that it takes only about or within one million generations for the transition to occur.

Perhaps we could agree that all living things are complex in their own right, right? Well, for me every living thing -- either single celled or multicellular organism -- is not as simple as it seems. A single-celled organism such as a bacterial cell is not simple but a complex organism. It may be a lone cell but that single cell can perform vital biological processes (like reproduction, digestion, respiration, etc.) that a typical human body would need millions of cells to accomplish.

Definitely, multicellular organisms are complex living things, too. Gavrilets assumes though that multicellular organisms have higher complexity since they are comprised of different cells committed to do specific roles. And they work together to survive and reproduce collectively as an individual. Gavrilets, thus, hoped to find out how and why the complexity has increased in the course of evolution.

Some of the important transitions in the evolution of complexity, i.e. in connection with the origin of chromosomes, eukaryotes, sex, multicellular organisms, and social groups in insects, had already been disclosed. Gavrilets believes that the transition toward division of labor is crucial to these transitions.

For Gavrilets, understanding why the division of labor evolved is quite difficult. Why would a single-celled unit be willing to take a role in a complex unit? Single-celled organisms are independent organism. They can thrive, survive, and reproduce on their own. Why would then a lone cell be "willing to share" the available space and nutrients with other cells when it can function on its own? Thus, the answer remains unclear. However, Gavrilets was able to provide a theoretical answer to the question "how".

How?

To find out how transition toward differentiation in a colony of cells emerged, he began with a colony of undifferentiated cells of volvocean green algae. Using mathematical modeling, he explored the possibility and the dynamics of the division of labor in relation to various factors, such as fitness advantage, mutation rate, costs of developmental plasticity, and the colony size.

The results were indeed interesting. The colony of undifferentiated cells eventually emerged as a multicellular organism where one portion of the colony eventually took the reproductive role while the other part eventually performed functions essential to survival. This is the first model that was able to show the evolution of complete germ-soma differentiation.1 Gavrilets' model can also tell which conditions would lead to this transition. It appears that two conditions are necessary to cause this transition. First is the presence of a strong genetic relatedness and the second is fitness trade-offs that prevent individual cells to perform multiple functions effectively. His model also tells us that this shift was rather rapid, happening already within one million generations.